Texture paint without affecting other object with same texture/material
Select 'Face Selection Masking' from the icon on the view header just to the left of the layer dialogue and that will limit by selection in the Texture Paint mode. You can make use of Vertex Groups here to go back and forth with selections, or you can use the 'L' key to select linked mesh while still in texture paint mode.

edit: If the image area falls under the same as another part, then the shared pixels will show no matter what. Your uv map has to give separate space for all mesh parts similar to Smart Unwrap in order to keep from painting two areas at once in the mapping.